How to easy flip/invert objects?

Objects that curve in a certain direction, like certain walls, helixes, stair cases etc. If I want to invert them so that they curve in the opposite direction, how can I easily accomplish this?

You can actually invert them using scale. You can even invert full groups. Say I'm building something that is symmetrical. I can build one side of it, group those pieces and then duplicate that group (using Cntrl - W) and then invert that group by setting the appropriate X, Y, Z to -1 instead of 1. Then I drag it into place. The mesh might look funny when you first do it, so just go into the group and move the meshes and hit Cntrl - Z to undo the move. This will fix their look.

Exactly what I was looking for! Thank you!